This invention relates to magnetic resonance imaging (MRI) and, more particularly, to a method and apparatus for improving image contrast, based upon intermolecular interactions with hydrogen nuclei spins.
MRI has largely supplanted X-ray imaging in a variety of soft tissue imaging applications, notably in the brain. However, contrast is largely based on variations in spin density or relaxation times, sometimes enhanced by injected contrast agents such as gadolinium. For the most part, these parameters are only loosely related to tissue morphology. Thus, it is not surprising that in many applications, no combination of these parameters gives sufficient useful contrast. Even with brain imaging, particularly in the rapidly expanding field of functional MRI, contrast is frequently the limiting factor. As another example, gadolinium-enhanced breast MRI can detect some tumors that mammograms miss, but the number of false positives is also high and MRI scans are far more expensive than mammograms. New methods for contrast enhancement could thus improve soft tissue characterization, particularly if they correlate with physiologically important characteristics.
As is known, every nuclear spin has a magnetic dipole moment, so it acts like a little bar magnet and affects all other spins. For instance, the NMR spectrum of a test tube of ice is 100 kHz wide due to the multiple interactions of the hydrogen spins in their relatively fixed positions. By contrast, the NMR spectra of a test tube of water is about 1 Hz wide because, in solution, diffusion of the dipoles causes the interactions to average, resulting in a substantial decrease in overall interaction.
It has been found possible to detect strong signals from intermolecular resonance effects. For example, the transition which corresponds to simultaneously flipping a water spin at one location and a water spin 100 microns away, in opposite directions has been seenxe2x80x94even though such an intermolecular xe2x80x9cmultiple quantum coherencexe2x80x9d would be predicted to be completely impossible to observe in the conventional formulation of an NMR system. Such a finding is set forth in the following papers: He et al., xe2x80x9cIntermolecular Multiple-Quantum Coherences and Cross-Correlations in Solution NMRxe2x80x9d, Journal of Chemical Physics, Volume 98, pp. 6779-6800 (1993); Richter et al., xe2x80x9cImaging with Intermolecular Multiple-Quantum Coherences in Solution NMRxe2x80x9d, Science, Volume 267, pp. 654-657 (1995); Vathyam et al., xe2x80x9cHomogeneous NMR Spectra in Homogeneous Fieldsxe2x80x9d, Science, Volume 272, pp. 92-96 (1996); and Lee et al., xe2x80x9cQuantum Treatment of the Effects of Dipole-Dipole Interactions in Solution NMRxe2x80x9d, Journal of Chemical Physics, Volume 105, pp. 874-900 (1996).
It is an object of this invention to provide an improved magnetic resonance imaging method which uses these intermolecular multiple-quantum coherences to generate contrast because of local variations in the magnetic susceptibility.
An improved method for magnetic resonance imaging of a sample includes the following steps: producing a magnetic field to orient the magnetic dipoles of a sample along the Z axis of an X,Y,Z Cartesian coordinate system; applying a first radio frequency (rf) pulse along a different axis, near the resonance frequency of the magnetic dipoles (e.g., of hydrogen atoms in water); applying a gradient pulse for a time tg in order to modulate the magnetization along a preferred axis in space (for example, the Z axis); allowing the magnetization to evolve for a time  zq, during which time the local variations in susceptibility affect the excited spins; applying a second radio frequency pulse near the resonant frequency of the magnetic dipoles; allowing the sample to evolve for a second time interval to create observable magnetization; and detecting this magnetization, using gradients to spatially resolve the signal, as is done in conventional imaging.